Ethylene vinyl alcohol (EVOH) copolymers are now recognized to have considerable value in packaging films, especially those that are intended for use for foods and other products that require protection against oxygen. The EVOH films do, however, suffer from a number of drawbacks. In particular, films made of EVOH tend to lack toughness and to be brittle particularly in the thin sections in which they are most economically used. Moreover, they do not provide an effective barrier against water or water vapor; in fact, the oxygen barrier properties of EVOH films decrease dramatically under conditions of relatively high humidity.
To accommodate such deficiencies and still take advantage of their attributes, the EVOH resins are normally used in multilayer films, wherein the EVOH layer is laminated to one or more structural layers, such as may be provided by nylon, polyethelene, polypropylene, and the like, and to various heat sealing layers, exemplary of which are the ionomer resins, ethylene vinyl acetate copolymers, etc.
However, because of the relatively poor flow characteristics exhibited by the EVOH copolymers in the softened and fused states, other difficulties are encountered when they are utilized in such multilayer films. Thus, it is found to be most difficult to achieve a uniform caliper across the entire width of the web, and this is particularly true when the EVOH is in contact with the other resins within the extruder die for a relatively long period of time. Also, inherent characteristics of the EVOH layer will preclude the use of such films for certain applications, and especially when the film is to be subsequently formed, as by vacuum thermoforming, which tends to cause small voids or so-called "pin-holes" to develop therein.
Accordingly, it is a primary object of the present invention to provide a novel EVOH blend which can be extruded to produce composite films of uniform caliber, which films may exhibit a reduced tendency to form pin-holes, splits and the like when subjected to thermoforming operations or abuse.
It is also an object of the invention to provide such compositions which, when formed into films, exhibit excellent oxygen barrier properties, high levels of clarity and freedom from haze, and structural properties that are improved in one or more respects.
Another object of the invention is to provide such compositions which are capable of being extruded at reduced temperatures and/or with reduced power requirements, thereby enhancing the economy of conversion.
Additional objects of the invention are to provide novel processes by which such compositions may be coextruded with other thermoplastic resinous materials, and to provide novel products thereof.